The synovial joints are the most common types of joints in mammals, provide free movement between the bones they join, and are typical of nearly all limb joints. They can be compared to mechanical bearings in a musculo skeletal machine. A synovial joint is the meeting point of two bones, movably arranged in relation to each other. The end surfaces of said bones are usually smooth and rounded, and covered by articular cartilage. A synovial membrane encapsulates the joint, forming a joint cavity, which contains synovial fluid. Outside the synovial membrane is a fibrous capsule and ligaments, forming an articular capsule.
A healthy joint is remarkably effective with coefficients of friction lower than those obtainable with man-made journal bearings (frictional bearings). Furthermore, the constant process of renewal and restoration of living tissue ensures that a synovial joint have a durability far superior to that of any artificial bearing. So far, no artificial joint can equal the performance of a normal human joint.
There are however both natural and pathological processes leading to deteriorated joint function. With age and wear, the articular cartilage becomes less effective as a shock absorber and a lubricated surface. Different degenerative joint diseases, such as arthritis, osteoarthritis, or osteoarthrosis, accelerate the deterioration.
Developments in material science, together with modern surgical techniques have made it possible to replace one or more of the contact surfaces, or the entire joint. Due to their weight-carrying function, hip and knee joints are most frequently addressed by surgical intervention and implantation of artificial components, or joint replacement surgery.
The lubrication of a healthy joint has been the focus of many researchers. Articular cartilage is elastic, fluid-filled, and backed by a relatively impervious layer of calcified cartilage and bone. This means that load-induced compression of cartilage will force interstitial fluid to flow laterally within the tissue and to surface through adjacent cartilage. As that area, in turn, becomes load bearing, it is partially protected by the newly expressed fluid above it. This is a special form of hydrodynamic lubrication, so-called because the dynamic motion of the bearing areas produces an aqueous layer that separates and protects the contact points.
Boundary layer lubrication is the second major low-friction characteristic of normal joints. Here, the critical factor is proposed to be a small glycoprotein called lubricin. The lubricating properties of this synovium-derived molecule are highly specific and depend on its ability to bind to articular cartilage where it retains a protective layer of water molecules. Lubricin is not effective in artificial systems and thus does not lubricate artificial joints.
Other lubricating mechanisms have been proposed; some remain under investigation. Interestingly, hyaluronic acid, the molecule that makes synovial fluid viscous (synovia means “like egg white”), has largely been excluded as a lubricant of the cartilage-on-cartilage bearing. Instead, hyaluronate lubricates a quite different site of surface contact-that of synovium on cartilage. The well-vascularized, well-innervated synovium must alternately contract and then expand to cover non-loaded cartilage surfaces as each joint moves through its normal range of motion. This process must proceed freely. Were synovial tissue to be pinched, there would be immediate pain, intraarticular bleeding, and inevitable functional compromise. The rarity of these problems testifies to the effectiveness of hyaluronate-mediated synovial lubrication.
WO 01/85179 discloses fluid compositions and methods for lubrication of mammalian joints are disclosed, including both natural and artificial fluids. Synovial fluid acts to lubricate the bearing surfaces of bones and bone-like structures which are held in frictional contact within biological joints. Such fluids may be used to treat arthritic, injured, and diseased joints. Synovial fluid containing a dextran-based hydrogel with lipids provides enhanced rheological and tribological properties of such a fluid. Phospholipids are particularly useful in dextran-based compositions for synovial fluid. One phospholipid that can be used advantageously in synovial fluid is dipalmitoyl phosphatidylcholine (DPFC).
Su et al. (Design and Mechanics Simulation of Bionic Lubrication System of Artificial Joints, Journal of Bionic Engineering, Volume 3, Issue 3, 2006) describe a new structure for artificial joints with a joint capsule which is designed to overcome the drawback of current prostheses that omit many functions of the lubricant and the joint capsule. The new structure is composed of three component: lubricant, artificial joint and artificial joint capsule. The lubricant sealed in the capsule can not only reduce the wear of the artificial joint but also prevents the wear particles leaking into the body. Thus, unexpected reactions between the wear particles and body can be avoided completely.
Radin et al. (Joint Lubrication with Artificial Lubricants, Arthritis & Rheumatism, 2005, Vol. 14, 1, 126-129) studied the joint lubricating properties, in vitro, in bovine metatarsal-phalangeal joints, of silicone fluid, methyl cellulose and polyvinyl-pyrrolidone compared to buffer, serum and synovial fluid. As has been previously reported, synovial fluid was almost twice as good as serum and buffer, which are equivalent in their joint lubricating qualities. Among the three artificial lubricants tested, only polyvinyl-pyrrolidone was superior to buffer or serum as a joint lubricant at 37° C. At 55° C., both polyvinyl-pyrrolidone and methyl cellulose had the same effect. At no time was any of the artificial lubricants tested as effective at reducing joint friction as was synovial fluid. Silicone fluid was consistently an inferior joint lubricant compared with buffer or serum. It was concluded that effective joint lubrication with artificial lubricants depends on their boundary and hydrophilic properties, rather than directly on their flow characteristics
Lubricants can be divided into three groups; gaseous, liquid and solid. For the purposes of this description, a solid lubricant is defined as a lubricant being solid and substantially maintaining its shape at body temperature and at a pressure and mechanical stress encountered in the mammal body, including in the joints of a mammal body.
Most solid lubricants are produced as thin solid films on sliding surfaces. They are also used as fillers in self-lubricating metallic, ceramic, and polymeric composites. In most cases, a transfer film is found on the sliding surfaces. For solid lubricant films, strong adhesion is key for long service life.
Boric acid (H3BO3) films, which provide the component surfaces with a self-replenishing solid lubricant, are formed from the reaction of the B2O3 surface (deposited by various conventional methods) on the component surface with the water present in the body of the recipient-patient. Conventional methods that can be employed to deposit either a boron, H3BO3, or B2O3 film on the annuloplasty ring component surface include vacuum evaporation (with or without ion bombardment) and simple oven curing of a thin layer over the implant surface. The self-lubricating mechanism of H3BO3 is governed by its unique layered, triclinic crystal structure which allows sheets of atoms to easily slide over each other during movement, thus minimizing component wear and friction.
When present at a sliding surface, solid lubricants function the same way as their liquid counterparts. Specifically, they shear easily to provide low friction and to prevent wear damage between the sliding surfaces. Several inorganic materials (e.g. molybdenum disulfide, graphite, hexagonal boron nitride, boric acid) can provide excellent lubrication. Most of these solids owe their lubricity to a lamellar or layered crystal structure. A few others (e.g. soft metals, polytetrafluoroethylene, polyimide, certain oxides and rare-earth fluorides, diamond and diamond-like carbons, fullerenes) can also provide lubrication although they do not have a layered crystal structure.
Certain polymers are also used as solid lubricants because the attractive properties they combine are unavailable in other solid lubricants. Polymers are particularly favored for applications where cost, weight, corrosion and biocompatibility are the major considerations. In short, solid lubricants have been around for a long time, and they have been meeting some very important and critical tribological needs.
UHMW PE is another polymer used widely in total joint replacement (Kurtz et al., 1999). Because of the very long molecules and highly entangled molecular chains, it provides better wear resistance than PTFE. However, wear of this polymer still poses a major obstacle for the longevity of the total joint replacement. Recent effort to solve these problems have increased interest in the structure, morphology, and mechanical properties of the UHMW PE and in various surface ad structural treatment processes (such as crosslinking).
(Solid lubricants and self-lubricating films, Bharat Bhushan, Modern tribology handbook, Vol. 1, 2000)
Hyaluronan or hyaluronic acid is approved by the FDA for the treatment of osteoarthritis in a method called visco supplementation. In this treatment, hyaluronan is injected through the articular capsule and the synovial membrane, into the joint cavity, supplementing the synovial fluid. While mechanically cushioning the joint, and providing a temporary analgesic effect, this treatment is nevertheless recommended only as a last alternative to surgery. The injection is difficult to perform, and painful.
The present inventor set out to develop an implantable device and method for the lubrication of joints, in particular synovial joints, including natural joints, joints comprising artificial component following partial joint replacement surgery, and complete artificial joints, following complete joint replacement surgery.
Preferably said solid lubricant comprises hyaluronan (hyaluronic acid) and optionally suitable additives. Hyaluronan is particularly preferred, as this is a nontoxic, noninflammatory biodegradable natural substance.
Hyaluronan is available in different qualities, such as relating to purity, molecular weight and degree of crosslinking. With regard to molecular weight, many different qualities are available, ranging from low molecular weight (LMW) or about 50,000 Da to high molecular weight (HMW) or about 4-6,000,000 Da. An increase in molecular weight results in corresponding increase in viscosity, from an oily liquid to a gel-like semisolid.
For example WO 01/60868 discloses single phase gels for preventing the formation of surgical adhesions. The gels are prepared by reacting an aqueous solution of a polyanionic polysaccharide, such as hyaluronic acid or carboxymethyl cellulose, with divinyl sulfone, to form a gel, the solution is neutralized, and a solid is precipitated from the solution. The solid can be redissolved in water to form a gel having properties which can be modified to suit a particular application. Using a similar approach, a hyaluronic acid containing solid can be produced, and inserted in contact with the articular surfaces in a joint, where the surrounding aqueous body fluids redissolve the solid, releasing hyaluronic acid to lubricate the joint.
US 2009181058 discloses an injectable or implantable rod-shaped formulation for delivery of osteogenic proteins to treat osteoporotic and/or osteopenic bone are disclosed. The formulation comprises hyaluronic acid derivatives and osteogenic proteins, and optional excipients and active ingredients such as a bone resorption inhibitor.
WO 2006/034383 discloses viscoelastic compounds encompassing any compound having viscoelastic properties including, but not limited to, cellulose polymers and their derivatives (for example hydroxypropyl methyl cellulose) and polysaccharides including, but not limited to, glucosaminoglycans such as hyaluronic acid and synthetic linear polymers. By way of example, the viscoelastic compound may be chondroitin sulphate, polyacrylamide, collagen, pectin, synthetic polymer-modified carbohydrate, hyaluronic acid or salts or esters thereof in essentially pure form and dry form, or mixtures of two or more of these compounds.
Suitable sodium hylauronates may in one aspect have a molecular mass of at least 5-6 million before sterilization which when dissolved to a 1% (w/w) solution will obtain similar characteristics as Healon® ophthalmic viscoelastic solution (OVD) (available from Abbot Medical Optics, Inc., Santa Ana, Calif.), or when dissolved to 2.3% (w/w) will resemble Healon® 5′ OVD (available from Abbot Medical Optics, Inc., Santa Ana, Calif.)). The preparation and purification of this type of sodium hyaluronate and to generate viscoelastic solutions are described in more detail in U.S. Pat. Nos. 4,141,973 and 6,086,697. Also high viscosity, high molecular mass sodium hyaluronates such as those described in U.S. Pat. No. 5,681,825 (marketed as visco elastic under the trade name Healon® GV) can be used with the present invention. One of ordinary skill in the art will realize that in other aspect of the invention, suitable sodium hylauronates may have a lower molecular mass, as low as 100,000 Da. Clearly, the desired molecular weight is dependent on the class of polymer that is desired to be used in association with the present invention. By way of example, and not of limitation, suitable viscoelastic solutions may be formed using HPMC in the weight range of from about 30,000 to about several hundred thousand daltons. Similarly, suitable viscoelastic solutions may be formed using chondroitin sulphate in the weight range starting from about 20,000 to about 30,000. In general, the molecular weight of the chosen visco elastic compound (whether it is sodium hyaluronate, HPMC or another viscoelastic) will be selected based on the desired visco elastic properties of the final solution.
Hip joint Osteoarthritis is a syndrome in which low-grade inflammation results in pain in the hip joints, caused by abnormal wearing of the Cartilage that act as a cushion inside if the hip joint. This abnormal wearing of the cartilage also results in a decrease of the joints lubricating fluid called Synovial fluid. Hip joint Osteoarthritis is estimated to affect 80% of all people over 65 years of age, in more or less serious forms.
The present treatment for hip osteoarthritis comprises NSAID drugs, local injections of Hyaluronic acid or Glucocorticoid to help lubricating the hip joint, and replacing parts of the hip joint with a prosthesis through hip joint surgery.
The replacing of parts of the hip joint is one of the most common surgeries to date performed at hundreds of thousand of patients in the world every year. The most common method comprises placing a metal prosthesis in Femur and a plastic bowl in Acetabulum. This operation is usually done through a lateral incision in the hip and upper thigh and through, Fascia Lata and the lateral muscles of the thigh. To get access to the hip joint, the supporting hip joint capsule attached to Femur and Ilium of Pelvis needs to be penetrated, making it difficult to get a fully functional joint after the surgery. Femur is then cut at the neck with a bone saw and the prosthesis is placed in femur either with bone cement or without. Acetabulum is slightly enlarged using an Acetabular reamer, and the plastic bowl is positioned using screws or bone cement.
The surgery typically requires one week of hospitalization due to the increased risk of infection. The recovery process is on average about 6 weeks, but even after this period the patient should not perform any physical activates that places large strain on the joint.